When a long segment of the human body is imaged using the conventional screen-film technique, special cassettes and films of extended length are used, such as 30×90 cm and 35×105 cm. As medical institutions are migrating from analog screen-film systems to digital modalities, such as computed radiography (CR), these types of exams impose a significant challenge. This is because the size of digital detector is limited. For example, the largest CR storage phosphor cassette from several major CR vendors is limited to 35×43 cm, which can only image a portion of the long body part at a time. To address this problem, several methods have been proposed. European Patent EP0919856A1 discloses a way of staggering several storage phosphor cassettes together. The cassettes can be in the alternating (FIG. 1A), staircase-wise (FIG. 1B), or oblique (FIG. 1C) arrangement. During the x-ray exposure, all the partially overlapping cassettes are exposed simultaneously, therefore each storage phosphor screen that resides inside the corresponding cassette records a part of the image of the long body part. The drawback of this approach is that the metallic frames of the front (closer to the x-ray source) cassettes impose shadows in the image recorded in the back cassettes. The shadows are not removable and therefore may hinder diagnostic interpretation of the acquired images. European Patent EP0866342A1 (also U.S. Pat. No. 5,986,279, issued Nov. 16, 1999, inventor Dewaele) presents a method that is based on partially overlapping a plurality of storage phosphor screens for extended imaging coverage. The screens can also be configured in an alternating (FIG. 1D), staircase-wise (FIG. 1E), or oblique (FIG. 1F) overlapping arrangement. Further, the screens can be contained in a single, extended length cassette for convenience of use. This approach overcomes the drawback of the cassette stacking method because there are no cassette metallic frames present in the x-ray path. However, in practice, this method requires that the storage phosphor screens be removed from the cassettes before imaging, and to be placed back into the cassettes in a darkroom after the x-ray exposure, which is cumbersome in the clinical environment.
The sub-images acquired by the individual storage phosphor screens must be stitched together to create a composite full image. The stitched full image should be distortion-free for the purposes of diagnostic interpretation and geometric measurement. U.S. Pat. No. 4,613,983, issued Sep. 23, 1986, inventors Yedid et al., discloses a method to reconstruct a composite radiographic image from a set of sub-images. However, this method is applicable only when the relative position between the sub-images is precisely controlled by the acquisition hardware. For any of the configurations shown in FIG. 1, the variation in the placement of the cassettes/phosphor screens during the x-ray exposure or the variability of the CR reader in scanning the phosphor screens, causes non-deterministic translation and rotation displacements between the acquired sub-images. The displacements can vary slightly from one exam to the next; geometric compensation is required to correct for the rotation and translation displacements. In addition, geometric compensation is also required to correct for magnification distortion caused by variations in distance from the x-ray source to the cassette and storage phosphor screens. To address these problems, European Patent EP0919858A1 proposes a method that utilizes a pattern of reference markers that impose shadows simultaneously with the diagnostic image in each of the acquired sub-images. After the reference markers are identified in each sub-image, the image distortion is corrected based on the known marker locations. Translation and rotation displacements between the sub-images are also computed using the known marker locations. Once the geometric compensation processing is completed, the composite full image is reconstructed. The drawback of this method is that a precisely fabricated pattern of reference markers must be imaged simultaneously with the patient in order to achieve precise geometric registration of the sub-images. The shadow of the reference markers may obscure diagnostically important information in the stitched image.
It is therefore desirable to develop an image processing algorithm that can (1) automatically perform image demagnification, (2) automatically detect and correct the translation and rotation displacements between the sub-images, and (3) form a composite full image that has high geometric fidelity without relying on external reference markers.